The present invention generally relates to optical disk apparatus, and move particularly to an optical disk storage apparatus having a tracking servomechanism.
In a tracking servomechanism of a conventional optical disk storage apparatus, a tracking servosignal is obtained by means of the push-pull method using a distribution of light bundles diffracted at each of guide grooves on an optical disk. In this method, a servo loop is closed to make the optical head follow the track after an optical actuator moves an optical head to the predetermined track. It takes about 200 ms until the mechanical oscillation of the optical actuator is damped.
However, the above tracking servomechanism has the following disadvantages as it does not consider the eccentricity of the disk. Because of a gap of even up to 100 .mu.m between the center of the disk and the rotational center, which is caused by the eccentricity of the center hole of the disk tracking errors increase during recording and/or reproducing, and/or it takes much time to access a target track. Hereupon, the gap of up to 70 .mu.m between the center of the disk and the rotational center can be permitted. FIG. 1(A) shows a rotating eccentric disk. As shown in the drawing, the center 2a of the disk 2 does not correspond to the rotational center 0. The disk rotates 180.degree. in the course of time T from T=t to T=t". FIG.1(B) shows the frequency characteristics of the tracking error signal of the disk shown in FIG.1(A). Because of the eccentricity of the disk, the frequency of the tracking error signal cannot be constant. Needless to say, the more eccentricity the disk has, the higher the frequency of the tracking error signal becomes. At a point A.sub.1 where the center 2a of the disk, the rotational center 0 and the optical head 3 are aligned with one another, in which time T is equal to t', the head 3 is located in a tangential direction of a track on the disk 2 so that the frequency of the tracking error signal can be low. On the other hand, at a point A.sub.2 and a point A.sub.3, in which time T is equal to t or t", the head 3 crosses many tracks so that the frequency of the tracking error signal becomes high. Therefore, in the vicinity of the point A.sub.1, the period of the tracking error signal is so long that the head 3 can be made to follow the predetermined track by means of the feedback method before the head 3 crosses another track. Accordingly, the head 3 can be rapidly made to follow the predetermined track by means of one performance of feedback control. However, in the vicinity of the points A.sub.2 and A.sub.3, the period of the tracking error signal is so short that the head 3 crosses another track before the head 3 is made to follow the predetermined track by means of the feedback method. Accordingly, the feedback control should be repeated until the head 3 is made follow the predetermined track, which takes much time. In the conventional optical disk storage, since the servo loop has been closed without considering the frequency of the tracking error signal, the head cannot be easily made to follow the predetermined track in a short time.